I want to add that there is a very important
policy statement on each page in the Extensions area on the lower left side clearly spelling out clearly for us, as community members what our responsibilities are. If we are not confident with our responsibilities, we should seek assistance before proceeding with an extension.
There are two problems I see people make repeatedly:
1) The "kid in a candy store" phenomena - Forums are installed on top of newsletters on top of image galleries, then, several installs later, no one can logon to the front end and they don't know when it happened or why.
2) Something is installed. It doesn't work. But, there was no backup of the website and the database.
If you go slow and test between extensions, you can avoid the first problem. If you institute a good backup and restore process, you will avoid the second problem. Even with bad extensions, you can easily recover if you follow standard practices.
Hvanleeuwen and Larae –
I agree with the points you raise. It can be very tricky in an open environment like this to know with any certainty if an extension will work or not. It gets worse, too. Just consider the millions of combinations of extensions possible! Many websites have a unique blend of extensions – different than any other set installed. There is simply no practical way to begin to test each of the various options available and certify it will always work under varied conditions.
Therein lies both the beauty and the beast of an open architecture!
That is why the responsibility for that decision rests with us, as community members. And, as you both correctly point out, we are not necessarily as skilled as we need to be.
It probably would be helpful if a community member (or team of community members who wanted to pair up on such a challenge!

) volunteered to take a bit of time to write a simple methodology that explains for others a “best practice” of how to "select, install, test, verify, include or back out an extension.” That type of instruction would be of good service and could be stored in our FAQ.
Thanks for raising these issues! They require discussion. Good work.
Amy